Yep, my experiments show the behavior. You have found yet another bug in UDL. Too bad there have been no bug fixes or feature improvements in UDL in years, so even if you made an official bug report, I doubt anything would ever happen with it. (There are other longstanding UDL bugs dealing with comments and folds. FOr example, if you have one or more single-line comments at the beginning of the file, any folding that happens afterward might be off-by-one-or-more-lines from where it should be.)
As a workaround, I suggest not using the multi-line comment definition if you want to fold that section of text; instead, use Folding in Code Style 1 if you can – or Style 2 if you’ve already used Style 1 (and are able to have newlines or spaces before and after your { and } indicators ).